Methods of directly manufacturing high-precision molded glass articles such as lenses by heating a glass preform and molding it in a press to a prescribed shape and methods of manufacturing glass optical elements such as lenses by grinding and polishing molded products are widely employed as techniques permitting the mass production of high-precision glass products such as optical elements with high reproducibility and good production efficiency. In such press-molding methods, press-molding devices, molding conditions and the like vary widely depending on whether grinding and polishing processes are required after press-molding. However, they all have in common the production of a glass preform suited to each press-molding, followed by the reheating and pressing of the preform.
Methods of taking a prescribed quantity of the glass melt and plastically transforming it into molded glass gobs to obtain the above-described glass preform afford good production efficiency because the glass preform can be produced without ever cooling the glass that has been melted. One such example is the method of molding glass gobs employing the device disclosed in Japanese Unexamined Patent Publication (KOKAI) Heisei No. 8-81228. This device sequentially receives a continuously flowing glass melt on multiple receiving molds and molds the glass melt into glass gobs on the receiving molds while the glass melt is still within a temperature range permitting deformation. In this device, multiple receiving molds are provided on a turntable, the turntable is intermittently rotated, and glass gobs are molded consecutively from the glass melt.
Although the above-mentioned method of molding glass gobs is a manufacturing method affording good production efficiency, the following technical background and problems exist.
(1) Since the glass melt is continuously discharged through a flow nozzle, the supplying (referred to hereinafter as “casting”) of glass melt to the receiving molds does not work well when time is required to switch out the receiving molds. Accordingly, the time required for one intermittent rotation of the turntable (referred to hereinafter as the “delivery time”) is limited.
(2) The cast glass is subjected to acceleration during intermittent rotation of the turntable. This acceleration exerts a force on the glass, deforming the glass. When the amount of deformation is small, the glass returns to a gob shape when the turntable stops. However, when the amount of deformation is significant, distortion remains in the glass after it returns its original shape. This distortion remains in the molded glass gob and in the glass preform employed for press-molding, becoming a defect in the press-molded product. The greater the acceleration exerted on the glass and the greater the weight of the glass, the greater the tendency of the glass to distort.
(3) Since the receiving molds are re-used in the above-described method of molding glass gobs, there is an upper limit to the rotation angle from the position at which casting is conducted to the position at which the glass gob is removed (referred to hereinafter as “takeout”). Since the above-mentioned delivery time is also limited, once the number of times the turntable is intermittently driven in one rotation (called the “section number”) has been determined, the time from casting to takeout has also been determined. However, when this time is excessively short, the glass gob is taken out before it has adequately cooled, that is, while it is still deformable. When this happens, the glass gob is deformed during takeout or during delivery process after takeout, the glass gobs stick to each other, and defective product is obtained.
Thus, in methods in which a continuously flowing glass melt is sequentially received by multiple receiving molds and molded into glass gobs by the receiving molds while still at a temperature range at which the glass melt is deformable, the above-described technical background and problems exist. For this reason, problems do not occur when comparatively small glass gobs are produced at slow speed. However, when comparatively large glass gobs are produced, or even when comparatively small glass gobs are produced at rapid speed (with better production efficiency), it is difficult to manufacture a good product.
Accordingly, it is an object of the present invention is to provide a method permitting the manufacturing of glass gobs without defects in appearance from a glass melt, that is, good product with good production efficiency, even when producing comparatively large glass gobs or producing comparatively small glass gobs at high speed (with better production efficiency); and a device for molding glass gobs.
A further object of the present invention is to provide a method of manufacturing molded glass products using the above-mentioned glass gobs as preforms for producing press-molded articles such as lenses or press-molded articles that are made into optical elements by grinding and polishing process.
A still further object of the present invention is to provide a method of manufacturing optical elements by grinding and polishing molded glass products obtained by the above-described method.